Variable radius mirror

ABSTRACT

A variable radius mirror includes a mirror element having a deformable face with an outer surface incorporating a reflective element. The deformable face is deformable in response to a pressure applied by a pressure medium acting on an inner surface of the deformable face. A ring extends around a perimeter of the deformable face and protrudes from the inner surface of the deformable face. The mirror element further includes at least one of a plurality of steps recessed at different depths into the inner surface of the deformable face, a cooling cavity having a pair of manifolds between the outer surface and the inner surface of the deformable face, and a sidewall of the ring having a curved inner surface and a curved outer surface.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates generally to a variable radius mirrorand, in particular, to a variable radius mirror for use with laseroptics systems.

Description of Related Art

A variable radius mirror (VRM) is a form of an adaptive optic elementused in industrial laser applications to control the convergence and/ordivergence of a laser beam. A VRM is configured to vary a radius ofcurvature of a reflective mirror surface by deflecting or flexing adeformable face having the mirror surface via selective actuation of anactuator, such as via delivery of pressurized air to a cavity behind themirror surface, to control where a laser beam comes to focus as the beampropagates.

There are many competing performance requirements that govern design ofVRMs. A large curvature range is desirable in VRM design because itallows for more adjustment of focal position and spot size while makingthe overall system design more compact and lightweight. The requirementfor a large curvature range must be balanced with materialcharacteristics. In addition, VRMs used in high power laser opticssystems need to dissipate heat efficiently from the mirror surface.These performance requirements are balanced by material selection andphysical design to achieve the desired VRM characteristics.

Due to these and other design criteria, existing VRMs are associatedwith a number of structural and performance deficiencies. Achieving alarge curvature range is challenging and requires an increasingly thindeformable face. While it is desired to have the deformable face bendunder pressure in a spherical manner, such bending may happen only overa small portion in the center of the deformable face before the radiusof curvature departs in a parabolic or catenary manner at outer edges ofthe deformable face. An existing solution for correcting theseirregularities of the mirror surface is by contouring the back of thedeformable face with a complex 3D curve. However, complex 3D curves aredifficult to manufacture and verify for dimensional accuracy.

Stresses on the deformable face during manufacture of the VRM and/orfrom connecting the mirror cap having the deformable face onto a rigidmirror base can pass distortions through to the mirror surface. ExistingVRMs compensate for such distortions by combining the mirror cap and themirror base as a matched pair. However, due to high tolerancespecifications, it can be difficult to match the mirror cap with themirror base, especially if the mirror cap has a complex 3D curve on aninside surface of the deformable face.

Cooling channels are provided as close as possible to the mirror surfacein order to increase heat transfer. However, the pressure within thecooling channels and thermal gradients across the deformable face due toirregular cooling medium flow within the cooling channels can impartvarious irregularities to the mirror surface, thereby impacting thedesired performance of the VRM.

Conventional VRMs are typically made from copper alloys and/or stainlesssteel. These materials are selected for their ratio of stiffness toyield strength, which determines a degree to which the deformable facecan be deformed while still showing a full elastic recovery. In additionto increasing the mass of the VRM, these materials create a galvanicpotential across the cooling medium, thereby resulting in corrosion andscale buildup within the cooling channels. Conventional VRMs are alsomachined to have sharp outer edges which define stress rising cornersthat may reduce the service life of the VRM.

Accordingly, there is a need in the art for an improved VRM thataddresses these and other drawbacks and deficiencies associated withexisting VRMs.

SUMMARY OF THE DISCLOSURE

In accordance with some non-limiting examples or aspects of the presentdisclosure, provided is an improved VRM that may include a mirrorelement including a deformable face having an outer surface with areflective element. The deformable face may be deformable in response toa pressure applied by a pressure medium acting on an inner surface ofthe deformable face. The mirror element may further include a ringextending around a perimeter of the deformable face and protruding fromthe inner surface of the deformable face. The mirror element may furtherinclude at least one of: (A) a plurality of steps recessed into theinner surface of the deformable face, each of the plurality of stepsrecessed at a different depth relative to the inner surface of thedeformable face; (B) a cooling cavity between the outer surface and theinner surface of the deformable face, the cooling cavity comprising apair of manifolds fluidly connected to each other and each having afirst curved end wall spaced apart from a second curved end wall; and(C) a sidewall of the ring having a curved inner surface and a curvedouter surface.

In accordance with some non-limiting examples or aspects of the presentdisclosure, a VRM may have a base element and a mirror element connectedto the base element with a pressure cavity defined between the baseelement and the mirror element. The mirror element may have a deformableface having an outer surface with a reflective element. The deformableface may be deformable in response to a pressure applied by a pressuremedium within the pressure cavity acting on an inner surface of thedeformable face. The mirror element may further have a ring extendingaround a perimeter of the deformable face and protruding from the innersurface of the deformable face. The mirror element may further have atleast one of: (A) a plurality of steps recessed into the inner surfaceof the deformable face, each of the steps recessed relative to the innersurface of the deformable face at an increasing or decreasing depth in adirection away from a central axis; (B) a cooling cavity between theouter surface and the inner surface of the deformable face, the coolingcavity comprising a pair of manifolds each having a first curved endwall spaced apart from a second curved end wall, the manifolds fluidlyconnected to each other by a plurality of cooling channels extendingthrough the first curved end wall of each manifold; and (C) a sidewallof the ring having a curved inner surface and a curved outer surface.

In accordance with some non-limiting examples or aspects of the presentdisclosure, a variable radius mirror may include a base element and amirror element connected to the base element with a pressure cavitydefined between the base element and the mirror element. The mirrorelement may include a deformable face having an outer surface with areflective element. The deformable face may be deformable in response toa pressure applied by a pressure medium within the pressure cavityacting on an inner surface of the deformable face. The mirror elementmay further include a ring extending around a perimeter of thedeformable face and protruding from the inner surface of the deformableface. A sidewall of the ring may have a curved inner surface and acurved outer surface. A plurality of steps may be recessed into theinner surface of the deformable face, with each of the steps beingrecessed at a different depth relative to the inner surface of thedeformable face. A cooling cavity may be provided between the outersurface and the inner surface of the deformable face. The cooling cavitymay include a pair of manifolds fluidly connected to each other and eachhaving a first curved end wall spaced apart from a second curved endwall. The mirror element may be made from a metal material having anelastic modulus less than or equal to 100 GPa.

These and other features and characteristics of the present disclosure,as well as the methods of operation and functions of the relatedelements of structures and the combination of parts and economies ofmanufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of a VRM in accordance with somenon-limiting examples or aspects of the present disclosure;

FIG. 2 is an exploded side view of the VRM shown in FIG. 1 ;

FIG. 3 is a first side cross-sectional view of the VRM shown in FIG. 1 ;

FIG. 4 is a second side cross-sectional view of the VRM shown in FIG. 1;

FIG. 5 is top perspective view of a mirror element for use with a VRM inaccordance with some non-limiting examples or aspects of the presentdisclosure;

FIG. 6 is a side view of the mirror element shown in FIG. 5 ;

FIG. 7 is a bottom view of the mirror element shown in FIG. 5 ;

FIG. 8 is a detailed cross-sectional view of a plurality of steps on aninner surface of the mirror element shown in FIG. 5 ;

FIG. 9 is a side cross-sectional view of the mirror element shown inFIG. 5 ; and

FIG. 10 is a side cross-sectional view of the mirror element shown inFIG. 5 .

DETAILED DESCRIPTION OF THE DISCLOSURE

As used herein, the singular form of “a”, “an”, and “the” include pluralreferents unless the context clearly dictates otherwise.

Spatial or directional terms, such as “left”, “right”, “inner”, “outer”,“above”, “below”, and the like, relate to the invention as shown in thedrawing figures and are not to be considered as limiting as theinvention can assume various alternative orientations.

All numbers used in the specification and claims are to be understood asbeing modified in all instances by the term “about”. By “about” is meantplus or minus ten percent of the stated value, such as plus or minusfive percent of the stated value.

As used herein, “at least one of” is synonymous with “one or more of”.For example, the phrase “at least one of A, B, or C” means any one of A,B, or C, or any combination of any two or more of A, B, or C. Forexample, “at least one of A, B, and C” includes A alone; or B alone; orC alone; or A and B; or A and C; or B and C; or all of A, B, and C.

The term “includes” is synonymous with “comprises”.

As used herein, the terms “parallel” or “substantially parallel” mean arelative angle as between two objects (if extended to theoreticalintersection), such as elongated objects and including reference lines,that is from 0° to 5°, or from 0° to 3°, or from 0° to 2°, or from 0° to1°, or from 0° to 0.5°, or from 0° to 0.25°, or from 0° to 0.1°,inclusive of the recited values.

As used herein, the terms “perpendicular” or “substantiallyperpendicular” mean a relative angle as between two objects at theirreal or theoretical intersection is from 85° to 90°, or from 87° to 90°,or from 88° to 90°, or from 89° to 90°, or from 89.5° to 90°, or from89.75° to 90°, or from 89.9° to 90°, inclusive of the recited values.

As used herein, the term “and/or” refers to both or either of two statedpossibilities.

With initial reference to FIGS. 1-2 , a variable radius mirror (VRM) 100is shown in accordance with some examples or aspects of the presentdisclosure. The VRM 100 may be configured for use as an adaptive opticelement in optical systems. For example, the VRM 100 may be used inindustrial laser applications to control the convergence and/ordivergence of a laser beam. The VRM 100 generally has a mirror element200 and a base 300. The mirror element 200 may be removably connected tothe base 300, such as using a plurality of fasteners 302. The mirrorelement 200 is configured to vary a focus point of the laser beamreflected from a reflective surface on the mirror element 200.

With continued reference to FIGS. 1-2 , the mirror element 200 includesa deformable face 202 having an outer surface 204 and an inner surface206 (shown in FIGS. 3-4 ). A reflective mirror surface or element 207 ispositioned on at least a portion of the outer surface 204 and isconfigured to reflect the laser beam. The mirror element 200 isconfigured to vary a radius of curvature of the deformable face 202 bydeflecting or flexing the deformable face 202 in a direction along acentral axis 210 via selective actuation of an actuation mechanism, asdescribed herein. Deflection of the deformable face 202 in turn changesa radius of curvature of the reflective element 207 to control theconvergence, divergence, or collimation of the reflected laser beam. Insome examples or aspects, the actuation mechanism for deflecting thedeformable face 202 may be pressure applied by a pressure medium actingon the inner surface 206 of the deformable face 202. By varying theradius of curvature of the reflective element 207 via deflection of thedeformable face 202, a focus point of a laser beam can be controlled todiverge the beam, converge the beam, or collimate the beam.

In some examples or aspects, the mirror element 200 has a circularcross-sectional shape. For example, as shown in FIG. 1 , the deformableface 202 of the mirror element 200 has a circular shape with pointsalong an outer edge of the deformable face 202 being substantiallyequidistant from a central axis 210. In other examples or aspects, themirror element 200 may have an oval, elliptical, or oblong shape havingone or more axes of symmetry. In such examples or aspects, the centralaxis 210 may define an approximate midpoint of the deformable face 202.In further examples or aspects, the mirror element 200 may have anyother cross-sectional shape.

In some examples, the mirror element 200 may have a radius of 0.8 mconcave to 0.8 m convex, with a usable clear aperture of 5% to 90% of asurface area of the deformable face 202. The size of the mirror element200 is selected based on a desired application, such as the power of thelaser beam or a desired focal length.

With reference to FIGS. 3-4 , and with continued reference to FIGS. 1-2, the mirror element 200 has a ring 208 protruding away from thedeformable face 202. In some examples or aspects, the ring 208 extendsaround a perimeter of the deformable face 202 and protrudes away fromthe inner surface 206. In examples or aspects where the deformable face202 has a circular shape, the ring 208 is revolved around the centralaxis 210 and has a corresponding circular shape that is centered aroundthe central axis 210 and extends from the inner surface 206 at or nearan outer edge 212 of the deformable face 202. In examples or aspectswhere the deformable face 202 has an oval, elliptical, or oblong shape,the ring 208 has a corresponding oval, elliptical, or oblong shape thatextends from to the inner surface 206 at or near the outer edge 212. Insome examples or aspects, a sidewall defined by the ring 208 extends ina direction that is substantially perpendicular to a plane defined bythe deformable face 202. In other examples or aspects, the sidewalldefined by the ring 208 extends in an acute or obtuse angle relative tothe plane defined by the deformable face 208. A groove 209 is cut intothe ring 208. The groove 209 is continuous along an entire perimeter ofthe ring 208.

With continued reference to FIGS. 3-4 , a pressure cavity 306 is definedbetween the mirror element 200 and the base 300. The pressure cavity 306is in fluid communication with a pressure source 308 having a pressuremedium that is used to pressurize the pressure cavity 306 in order todeform the deformable face 202. In some examples or aspects, thepressure medium is a gas (such as air), or a liquid. The mirror element200 and the base 300 are connected together in a leak-proof manner toprevent escape of the pressure medium from the pressure cavity 306. Forexample, a gasket 304 may be provided at an interface between the ring208 and the base 300.

With reference to FIG. 3 , the base 300 has one or more passages 310having a first end in fluid communication with the pressure source 308and a second end in fluid communication with the pressure cavity 306.The pressure source 308 may include at least one of a storage tank forstoring the pressure medium, a pump for pressurizing the pressure mediumand/or delivering the pressure medium from the storage tank, and one ormore fluid lines for delivering the pressure medium from the storagetank and/or the pump to the base 300. In addition, one or more valvesmay be provided to control the flow of the pressure medium between thepressure source 308 and the pressure cavity 306.

In some examples or aspects, the base 300 may have one or more inletpassages 310 a for introducing the pressure medium from the pressuresource 308 into the pressure cavity 306 and one or more outlet passages310 b for exhausting the pressure medium from the pressure cavity 306.In other examples or aspects, the base 300 may have a single passage 310for introducing the pressure medium into the pressure cavity 306 andexhausting the pressure medium from the pressure cavity 306. One or morevalves (not shown) may be provided to control the flow of the pressuremedium into and out of the pressure cavity 308 via the one more passages310.

With continued reference to FIG. 3 , a pressure controller 312 controlsthe passage of pressure medium into and out of the pressure cavity 306.For example, the pressure controller 312 is configured to increase thepressure of the pressure medium within the pressure cavity 306 in orderto increase a deflection of the deformable face 202 and change itsradius of curvature or decrease the pressure of the pressure mediumwithin the pressure cavity 306 in order to decrease a deflection of thedeformable face 202. For example, the pressure controller 312 maycontrol the output of a pump or other pressurization system used topressurize the pressure medium and/or deliver the pressurized medium tothe pressure cavity 306. Desirably, the pressure controller 312 isconfigured to control the pressure of the pressure medium within thepressure cavity 306 without introducing perturbations into thereflective surface of the reflective element 206 which would otherwisedisturb focus of the laser beam. In some examples or aspects, thepressure controller 312 is configured to control the pressure within thepressure cavity 306 between 0 bar to 15 bar absolute pressure.

With continued reference to FIG. 3 , the mirror element 200 has acooling cavity 214 defined between the outer surface 204 and the innersurface 206 of the deformable face 202. The cooling cavity 214 is influid communication with a cooling medium source 314 having a coolingmedium that is delivered to the cooling cavity 214 to cool thedeformable face 202. In some examples or aspects, the cooling medium isa gas (such as air), or a liquid. The cooling cavity 214 has a pluralityof cooling channels 216 configured for flowing the cooling mediumtherethrough.

With reference to FIG. 4 , the base 300 has one or more cooling passages316 having a first end in fluid communication with the cooling mediumsource 314 and a second end in fluid communication with the coolingcavity 214. The cooling medium source 314 may include at least one of astorage tank for storing the cooling medium, a pump for delivering thecooling medium from the storage tank, and one or more fluid lines fordelivering the cooling medium from the storage tank and/or the pump tothe base 300. In addition, one or more valves may be provided to controlthe flow of the cooling medium between the cooling medium source 314 andthe cooling cavity 214.

In some examples or aspects, the base 300 may have one or more inletcooling passages 316 a for introducing the cooling medium into thecooling cavity 214 and one or more outlet cooling passages 316 b forexhausting the cooling medium from the cooling cavity 214. A coolingcontroller 318 controls the passage of the cooling medium into and outof the cooling cavity 214. For example, the cooling controller 318 isconfigured to increase or decrease a flow rate and/or pressure of thecooling medium delivered to the cooling cavity 214 in order to increaseor decrease a rate of heat dissipation from the deformable face 202. Insome examples or aspects, the cooling controller 318 is configured tocontrol the flow rate of the cooling medium delivered to the coolingcavity 214 between 0.01 L/min to 100 L/min. Desirably, the coolingcontroller 318 is configured to control the flow rate and/or pressure ofthe cooling medium flowing through the cooling cavity 214 withoutintroducing perturbations into the reflective surface of the reflectiveelement 207 which would otherwise disturb focus of the laser beam. Insome examples or aspects, the pressure controller 312 and the coolingcontroller 318 may be combined into a single device.

With reference to FIGS. 5-10 , a mirror element 200 is shown inaccordance with another example or aspect of the present disclosure.Certain components of the mirror element 200 shown in FIGS. 1-4 aresubstantially similar or identical to the components of the mirrorelement 200 described herein with reference to FIGS. 5-10 . Accordingly,reference numerals in FIGS. 5-10 are used to illustrate similar oridentical components of the corresponding reference numerals in FIGS.1-4 . As the previous discussion regarding the mirror element 200generally shown in FIGS. 1-4 is applicable to the mirror element 200shown in FIGS. 5-10 , only the relative differences between the twomirror elements 200 are discussed hereinafter.

With reference to FIGS. 5-6 , the ring 208 of the mirror element 200 hasa sidewall 220 with a curved outer surface 222 extending from an edge212 of the deformable face 202. The curved outer surface 222 may have aconstant radius of curvature or a variable radius of curvature. In someexamples or aspects, the curved outer surface 222 has a first endstarting at the edge 212 of the deformable face 202 and a second endterminating at the groove 209 on the ring 208.

With reference to FIG. 9 , the sidewall 220 further may have a curvedinner surface 224. The sidewall 220 thus may have a shape of an outersurface of a toroid. The curved inner surface 224 may have a constantradius of curvature or a variable radius of curvature. In some examplesor aspects, the curved inner surface 224 has a first end starting at anouter edge of the inner surface 206 of the deformable face 202 and asecond end terminating at the groove 209 on the ring 208.

In some examples or aspects, the curved inner surface 224 may have thesame shape as the curved outer surface 222 such that the sidewall 220has a constant or uniform thickness T₁ at least in the area between thecurved outer surface 222 and the curved inner surface 224. In otherexamples or aspects, the thickness of the sidewall 220 may increase ordecrease (i.e., thickness T₁ may be non-uniform) in a direction from theedge 212 of the deformable face 202 toward the groove 209 on the ring208. A lower portion 225 of the sidewall 220 below the groove 209 mayhave an increased thickness compared to the thickness the sidewall 220above the groove 209.

The curved sidewall 220 is configured to distribute the stress ofdeformation across a broad curve instead of focusing the stress in oneor more corners with a tight radius. In addition, the curved sidewall220 enables maximum deformation of the deformable face 202 even when themirror element 200 is made from a material with a lower yield point. Forexample, with the mirror element 200, including the deformable face 202and the ring 208, is made from a metal material with an elastic modulusless than or equal to 100 GPa, the deformable face 202 can be deformedto a greater degree (i.e., have a larger radius of curvature) whilestill showing a full elastic recovery. Furthermore, stresses on thedeformable face 202 from connecting the mirror element 200 onto the base300 (shown in FIGS. 1-2 ), such as due to errors in the mounting surfaceand torque variations between the fasteners, are absorbed by the curvedsidewall 220, thereby allowing the mirror element 200 to be manufacturedindependently of a matched base 300.

In some examples or aspects, the mirror element 200 shown in FIGS. 1-10is made from a metal material having an elastic modulus that is lessthan or equal to 100 GPa. For example, the mirror element 200 may bemade from aluminum or an aluminum alloy. Use of aluminum allows for areduction in weight of the mirror element 200 compared to use ofconventional materials, such as stainless steel and/or copper. Lowerweight in turn contributes to a faster movement of the deformable face202, thereby increasing the performance response of the mirror element200.

In contrast to conventional VRMs, which have mirror elements made frommetal materials with an elastic modulus greater than 100 GPa (i.e,stainless steel and copper), the mirror element 200 made from a metalmaterial with an elastic modulus less than or equal to 100 GPa allowsfor a greater deformation of the deformable face 202 while still showinga full elastic recovery. In addition, when used with VRMs having acooling cavity, the aluminum or aluminum alloy does not create agalvanic potential across the cooling medium, thereby preventingcorrosion and scale buildup within the cooling channels.

With reference to FIG. 9 , the deformable face 202 may have a thicknessT₂ of 0.5 mm to 30.0 mm measured between the outer surface 204 and theinner surface 206. The use of aluminum allows for a thicker deformableface 202 compared to a thickness of the deformable face used inconventional mirror elements (0.8 mm to 2.9 mm) while allowing a fullrange of deformation. With a thicker deformable face 202, deeper coolingchannels can be easily machined to increase the cooling capacity. Inaddition, increased thickness of the deformable face 202 allows foreasier fixturing and machining of the mirror element 200 duringmanufacture.

With reference to FIGS. 7-8 , the mirror element 200 has one or moresteps 226 recessed into the inner surface 206 of the deformable face202. Each step 226 has a rise 227, expressed as a distance measured in adirection along the central axis 210, and a run 229, expressed as adistance measured in a direction perpendicular to the central axis 210.Each step 226 may have the same rise 227 as the other steps 226 or adifferent rise 227 relative to the other steps. The rise 227 may besubstantially parallel to the central axis 210 or angled at an obtuse oracute angle relative to the central axis 210.

Regardless of the rise 227, the steps 226 are recessed at a differentdepth relative to the inner surface 206. The depth of the steps 226 mayincrease and/or decrease in a direction away from the central axis 210.For example, the depth of each step 226 may increase in a direction awayfrom the central axis 210, decrease in the direction away from thecentral axis 210, or the depth of at least one of a plurality of steps226 may increase while a depth of at least another one of the pluralityof steps 226 decreases in the direction away from the central axis 210.As shown in FIG. 8 , the depth of the steps 226 continuously increasesin a direction away from the central axis 210. The step 226 closest tothe central axis 210 may be in the same plane as the inner surface 206or recessed relative to the plane defined by the inner surface 206.

In some examples or aspects, such as shown in FIG. 7 , the steps 226 maybe formed as a series of concentric rings centered about the centralaxis 210. Each step 226 may have the same run 229 as the other steps 226or a different run 229 relative to the other steps. The run 229 or widthof the steps 226 may increase and/or decrease in a direction away fromthe central axis 210. For example, the run 229 or width of each step 226may increase in a direction away from the central axis 210, decrease inthe direction away from the central axis 210, or the run 229 of at leastone of a plurality of steps 226 may increase while a run 229 of at leastanother one of the plurality of steps 226 decreases in the directionaway from the central axis 210. The run 229 may be substantiallyperpendicular to the central axis 210 or angled at an obtuse or acuteangle relative to the central axis 210. The steps 226 may be continuousor discontinuous in a circumferential direction about the central axis210.

The steps 226 may be added during manufacture of the mirror element 200to correct any irregularities on the outer surface 204 which may preventdeformation of the deformable face 202 in a spherical or torroidalmanner. The steps 226 may be added to correct the figure of the deformedouter surface 204 away from the natural catenary shape to anoptically-desirable spherical shape. The steps 226 can be easilymachined into the inner surface 206 of the deformable face 202. A mirrorelement 200 having the steps 226 on the inner surface 206 of thedeformable face 202 can be mounted on a fixturing tool, such as a vacuumchuck, having corresponding steps. In this manner, vibrations of theouter surface 204 of the deformable face 202 during machining can beminimized or eliminated The geometry of the steps 226 is independent ofthe thickness and material of the deformable face 202, therebypermitting use of the steps 226 on any deformable face of sufficientthickness as long as the steps do not cause a stress rise above theyield point of the material.

With reference to FIG. 10 , the cooling cavity 214 has a pair ofmanifolds 228 fluidly connected to each other by the plurality ofcooling channels 216. Each manifold 228 has a first curved end wall 231spaced apart from a second curved end wall 233, with each of the coolingchannels 216 extending through the first curved end wall 231 of eachmanifold 228. In some examples or aspects, a curvature of the firstcurved end wall 231 and/or the second curved wall 233 may be selected tocorrespond to a curvature of the outer surface of the mirror element200.

The cooling cavity 214 is in fluid communication with a cooling mediumsource 314 (shown in FIG. 4 ) via at least one cooling cavity inlet 230and at least one cooling cavity outlet 232. The at least one coolingcavity inlet 230 may be provided in a first of the pair of manifolds 228and at least one cooling cavity outlet 232 may be provided in a secondof the pair of manifolds 228.

With continued reference to FIG. 10 , the plurality of cooling channels216 that fluidly connect the pair of manifolds 228 may be arrangedsubstantially parallel to each other. The length of the cooling channels216 depends on the shape of the manifolds 228. For example, as shown inFIG. 10 , cooling channels 216 that are radially closer to an edge ofthe mirror element 200 are shorter than the cooling channels 216 thatare closer to the middle of the mirror element 200 due to the curvedshape of the manifolds 228. In this manner, the longer cooling channelshave a larger heat absorption rate compared to the shorter coolingchannels 216 due to their larger surface area.

An efficient cooling medium flow within the cooling cavity 214 reducesthermal gradients across the deformable face 202 and irregularities inthe mirror surface which can impact the desired performance of the VRM100.

As shown in FIG. 8 , each cooling channel 216 has a width W and a heightH. The width W and height H of the cooling channels 216 may be uniform.In some examples of aspects, the width W and height H of the coolingchannels 216 may vary across the surface of the deformable face 202.Cooling channels 216 may be provided as close as possible to the outersurface 204 of the deformable face 202 in order to increase heattransfer and enable increased radius of curvature.

An increased thickness T2 of the deformable face 202 allows for widerand deeper cooling channels 216, thereby increasing the flow rate of thecooling medium at a given pressure. The flow rate is further improvedand regulated by the curved shape of the manifolds 228 which urge thecooling medium along the curved walls and into the cooling channels 216.

Further examples or aspects of a variable radius mirror are detailed inthe following numbered clauses.

Clause 1. A variable radius mirror comprising: a mirror elementcomprising: a deformable face having an outer surface with a reflectiveelement, the deformable face being deformable in response to a pressureapplied by a pressure medium acting on an inner surface of thedeformable face; a ring extending around a perimeter of the deformableface and protruding from the inner surface of the deformable face; andat least one of: a plurality of steps recessed into the inner surface ofthe deformable face, each of the plurality of steps recessed at adifferent depth relative to the inner surface of the deformable face; acooling cavity between the outer surface and the inner surface of thedeformable face, the cooling cavity comprising a pair of manifoldsfluidly connected to each other and each having a first curved end wallspaced apart from a second curved end wall; and a sidewall of the ringhaving a curved inner surface and a curved outer surface.

Clause 2. The variable radius mirror according to clause 1, wherein theplurality of steps are formed as a plurality of concentric ringscentered about a central axis

Clause 3. The variable radius mirror according to clause 1 or 2, whereinthe depth of the plurality of steps decreases in a direction away from acentral axis.

Clause 4. The variable radius mirror according to any of clauses 1-3,wherein the depth of the plurality of steps increases in a directionaway from a central axis.

Clause 5. The variable radius mirror according to any of clauses 1-4,wherein a width of the plurality of steps measured along the innersurface of the deformable face increases or decreases in a directionaway from a central axis, or wherein the width of at least one of theplurality of steps increases while the width of at least another one ofthe plurality of steps decreases in the direction away from the centralaxis.

Clause 6. The variable radius mirror according to any of clauses 1-5,wherein the cooling cavity further comprises at least one cooling inletin a first of the pair of manifolds and at least one cooling outlet in asecond of the pair of manifolds.

Clause 7. The variable radius mirror according to any of clauses 1-6,wherein the cooling cavity further comprises a plurality of coolingchannels fluidly connecting the pair of manifolds.

Clause 8. The variable radius mirror according to clause 7, wherein eachof the cooling channels extends through the first curved end wall ofeach manifold.

Clause 9. The variable radius mirror according to clause 7 or 8, whereinthe plurality of cooling channels are parallel to each other.

Clause 10. The variable radius mirror according to any of clauses 1-9,wherein a thickness of the sidewall between the curved inner surface andthe curved outer surface is uniform.

Clause 11. The variable radius mirror according to any of clauses 1-10,wherein the sidewall of the ring is shaped as an outer surface of atoroid.

Clause 12. The variable radius mirror according to any of clauses 1-11,wherein the mirror element is made from a metal material having anelastic modulus less than or equal to 100 GPa.

Clause 13. The variable radius mirror according to any of clauses 1-12,wherein the mirror element is made from aluminum or an aluminum alloy.

Clause 14. The variable radius mirror according to any of clauses 1-13,further comprising a base element connected to the ring of the mirrorelement, and a pressure cavity defined between the mirror element andthe base element.

Clause 15. The variable radius mirror according to clause 14, whereinthe base element has at least one passage in fluid communication withthe pressure cavity.

Clause 16. A variable radius mirror comprising: a base element; and amirror element connected to the base element with a pressure cavitydefined between the base element and the mirror element, the mirrorelement comprising: a deformable face having an outer surface with areflective element, the deformable face being deformable in response toa pressure applied by a pressure medium within the pressure cavityacting on an inner surface of the deformable face; a ring extendingaround a perimeter of the deformable face and protruding from the innersurface of the deformable face; and at least one of: a plurality ofsteps recessed into the inner surface of the deformable face, each ofthe steps recessed relative to the inner surface of the deformable faceat an increasing or decreasing depth in a direction away from a centralaxis; a cooling cavity between the outer surface and the inner surfaceof the deformable face, the cooling cavity comprising a pair ofmanifolds each having a first curved end wall spaced apart from a secondcurved end wall, the manifolds fluidly connected to each other by aplurality of cooling channels extending through the first curved endwall of each manifold; and a sidewall of the ring having a curved innersurface and a curved outer surface.

Clause 17. The variable radius mirror according to clause 16, whereinthe mirror element is made from a metal material having an elasticmodulus less than or equal to 100 GPa.

Clause 18. The variable radius mirror according to clause 16 or 17,wherein the base element has at least one passage in fluid communicationwith the pressure cavity.

Clause 19. The variable radius mirror according to any of clauses 16-18,wherein the cooling cavity further comprises at least one cooling inletin a first of the pair of manifolds and at least one cooling outlet in asecond of the pair of manifolds.

Clause 20. A variable radius mirror comprising: a base element; and amirror element connected to the base element with a pressure cavitydefined between the base element and the mirror element, the mirrorelement comprising: a deformable face having an outer surface with areflective element, the deformable face being deformable in response toa pressure applied by a pressure medium within the pressure cavityacting on an inner surface of the deformable face; a ring extendingaround a perimeter of the deformable face and protruding from the innersurface of the deformable face, a sidewall of the ring having a curvedinner surface and a curved outer surface; a plurality of steps recessedinto the inner surface of the deformable face, each of the stepsrecessed at a different depth relative to the inner surface of thedeformable face; and a cooling cavity between the outer surface and theinner surface of the deformable face, the cooling cavity comprising apair of manifolds fluidly connected to each other and each having afirst curved end wall spaced apart from a second curved end wall,wherein the mirror element is made from a metal material having anelastic modulus less than or equal to 100 GPa.

Although the disclosure describes what are currently considered to bethe most practical and preferred examples or aspects, it is to beunderstood that such detail is solely for that purpose and that thedisclosure is not limited to the disclosed examples or aspects, but, onthe contrary, is intended to cover modifications and equivalentarrangements that are within the spirit and scope of the appendedclaims. For example, it is to be understood that the present disclosurecontemplates that, to the extent possible, one or more features of anyexample or aspect can be combined with one or more features of any otherexample or aspect.

What is claimed is:
 1. A variable radius mirror comprising: a mirrorelement comprising: a deformable face comprising: an outer surface thatis reflective; and an inner surface that opposes the outer surface, theinner surface comprising: a central surface centered on a central axisof the variable radius mirror and extending in a plane; a peripheralsurface extending in the plane; and a plurality of steps between thecentral surface and the peripheral surface that are entirely recessedfrom the central surface and the peripheral surface towards the outersurface; and a ring extending around a perimeter of the deformable faceand protruding from the inner surface of the deformable face, whereinthe deformable face is configured to deform when a pressure from acontrolled pressure source is applied to the inner surface, and whereineach of the plurality of steps are recessed at different depths relativeto the inner surface of the deformable face.
 2. The variable radiusmirror according to claim 1, wherein the plurality of steps are formedas a plurality of concentric rings centered about a central axis.
 3. Thevariable radius mirror according to claim 1, wherein the differentdepths decrease in a direction away from a central axis.
 4. The variableradius mirror according to claim 1, wherein the different depthsincrease in a direction away from a central axis.
 5. The variable radiusmirror according to claim 1, wherein a width of the plurality of stepsmeasured along the inner surface of the deformable face increases ordecreases in a direction away from a central axis, or wherein the widthof at least one of the plurality of steps increases while the width ofat least another one of the plurality of steps decreases in thedirection away from the central axis.
 6. The variable radius mirroraccording to claim 1, wherein the mirror element is made from a metalmaterial having an elastic modulus less than or equal to 100 GPa.
 7. Thevariable radius mirror according to claim 6, wherein the metal materialis aluminum or an aluminum alloy.
 8. The variable radius mirroraccording to claim 1, further comprising a base element connected to thering of the mirror element, and a pressure cavity defined between themirror element and the base element.
 9. The variable radius mirroraccording to claim 8, wherein the base element has at least one passagein fluid communication with the pressure cavity.
 10. The variable radiusmirror according to claim 8, wherein a sidewall of the ring has a curvedinner surface and a curved outer surface; and wherein the mirror elementcomprises a cooling cavity between the outer surface and the innersurface of the deformable face, the cooling cavity comprising a pair ofmanifolds fluidly connected to each other and each having a first curvedend wall spaced apart from a second curved end wall.
 11. The variableradius mirror according to claim 1, wherein the mirror element comprisesa cooling cavity between the outer surface and the inner surface of thedeformable face, the cooling cavity comprising a pair of manifoldsfluidly connected to each other and each having a first curved end wallspaced apart from a second curved end wall.
 12. The variable radiusmirror according to claim 11, wherein the cooling cavity furthercomprises at least one cooling inlet in a first manifold of the pair ofthe manifolds and at least one cooling outlet in a second manifold ofthe pair of the manifolds.
 13. The variable radius mirror according toclaim 11, wherein the cooling cavity further comprises a plurality ofcooling channels fluidly connecting the pair of the manifolds.
 14. Thevariable radius mirror according to claim 13, wherein each of thecooling channels extends through the first curved end wall of each thepair of the manifolds.
 15. The variable radius mirror according to claim13, wherein the plurality of cooling channels are parallel to eachother.
 16. The variable radius mirror according to claim 11, furthercomprising a base element; wherein the mirror element is connected tothe base element with a pressure cavity defined between the base elementand the mirror element, wherein each of the plurality of steps isrecessed relative to the inner surface of the deformable face at anincreasing or decreasing amount of the different depths in a directionaway from a central axis; and wherein the manifolds are fluidlyconnected to each other by a plurality of cooling channels extendingthrough the first curved end wall of each the pair of the manifolds. 17.The variable radius mirror according to claim 16, wherein the mirrorelement is made from a metal material having an elastic modulus lessthan or equal to 100 GPa.
 18. The variable radius mirror according toclaim 16, wherein the base element has at least one passage in fluidcommunication with the pressure cavity.
 19. The variable radius mirroraccording to claim 16, wherein the cooling cavity further comprises atleast one cooling inlet in a first manifold of the pair of the manifoldsand at least one cooling outlet in a second manifold of the pair of themanifolds.
 20. The variable radius mirror according to claim 1, whereina sidewall of the ring has a curved inner surface and a curved outersurface.
 21. The variable radius mirror according to claim 20, wherein athickness of the sidewall between the curved inner surface and thecurved outer surface is uniform.
 22. The variable radius mirroraccording to claim 20, wherein the sidewall of the ring is shaped as anouter surface of a toroid.
 23. The variable radius mirror according toclaim 1, wherein the central surface of the inner surface and theperipheral surface of the inner surface are inner most surfaces of thedeformable face.
 24. A variable radius mirror comprising: a mirrorelement comprising: a deformable face comprising: an outer surface thatis reflective; and an inner surface that opposes the outer surface,wherein the deformable face is configured to deform when a pressure froma controlled pressure source is applied to the inner surface, the innersurface comprising: a central surface centered on a central axis of thevariable radius mirror and extending in a plane; a peripheral surfaceextending the plane; a plurality of steps between the central surfaceand the peripheral surface that are entirely recessed from the centralsurface and the peripheral surface towards the outer surface; and acooling cavity between the outer surface and the inner surface, thecooling cavity comprising a pair of manifolds fluidly connected to eachother and each having a first curved end wall spaced apart from a secondcurved end wall; and a ring extending around a perimeter of thedeformable face and protruding from the inner surface of the deformableface.
 25. The variable radius mirror according to claim 24, wherein thecooling cavity further comprises at least one cooling inlet in a firstmanifold of the pair of the manifolds and at least one cooling outlet ina second manifold of the pair of the manifolds.
 26. The variable radiusmirror according to claim 24, wherein the cooling cavity furthercomprises a plurality of cooling channels fluidly connecting the pair ofthe manifolds.
 27. The variable radius mirror according to claim 26,wherein each of the cooling channels extends through the first curvedend wall of each manifold of the pair of the manifolds.
 28. The variableradius mirror according to claim 26, wherein the plurality of coolingchannels are parallel to each other.